Some bosses have Thrash granting extra attacks, but because the combat log does not explicitly declare when a Thrash occurs, it is not a commonly-known boss mechanic and cannot be easily data-mined. The only indication of a boss having this ability is a series of hits all happening on the same time-stamp, which can only be seen by players with the right addons and UI configurations.

It is likely that Thrash (A) can proc from itself or (B) the number of extra attacks varies per boss, as players have witnessed a variety in the number of Thrash attacks coming from different bosses.